The invention relates to vestigial sideband (VSB) signal receivers and, more particularly, the invention relates to passband equalization circuits for such VSB signal receivers.
In a terrestrial broadcast channel, a transmitted signal experiences time dispersion due to a deviation in the channel frequency response from the ideal channel characteristics of a constant amplitude and linear phase (constant delay) response. Consequently, to achieve optimal demodulation of a terrestrially broadcast signal, an equalizer is required in the receiver system to compensate for the non-ideal channel characteristics by using adaptive filtering. By correcting the amplitude and phase response of the received signal, the equalizer minimizes the intersymbol interference (ISI) of the received signal, thus improving the signal detection accuracy.
In conventional high definition television (HDTV) receivers using 8-VSB formatted data, the equalization process is accomplished at baseband, i.e., xe2x80x9crealxe2x80x9d equalization is accomplished after carrier and timing recovery is performed. To aid the equalization process, the data includes a data field sync signal (DFS) that repeats within the data every 24.2 milliseconds. The DFS contains a known pseudorandom bit sequence which is used as a training pattern in order to aid the equalizer in achieving an initial set of filter tap settings during start-up and after a channel change. After the system start-up process is performed using the training data, the xe2x80x9ceyexe2x80x9d within the xe2x80x9ceyexe2x80x9d pattern of the data is fairly open and symbol sampling is very accurate.
Once the start-up process is complete, the equalizer switches itself to a decision directed mode, which then tracks the slow changes in the channel response characteristics. In practice, sudden changes in the channel response characteristics (i.e., generally caused by the user changing channels) can drastically change the characteristics of the VSB signal being received, disabling the equalizer while it is operating in the decision directed mode. When this happens, the equalizer will distort the received signal since it does not compensate for the current channel response, i.e., the equalizer filter tap settings are defined for the previously selected channel. As such, the receiver begins making incorrect symbol decisions requiring the equalizer to be switched back to the training mode to achieve proper synchronization. However, to achieve proper synchronization in the training mode, the equalizer must wait until the next few DFS bit sets become available as the equalizer requires multiple DFS to achieve proper equalization for the newly selected channel. Even in the most optimistic situation where only one DFS bit set is necessary to achieve equalization, the wait would be at least 24.2 milliseconds.
Therefore, a need exists in the art for a passband equalization circuit that does not rely on the DFS to perform initial equalization such that the speed at which an equalizer reestablishes equalization is substantially improved.
The disadvantages associated with the prior art are overcome by the present invention of a passband equalizer that performs blind equalization upon a vestigial sideband (VSB) signal without using a training sequence. The passband equalizer contains a feed forward equalizer (FFE) and a decision feedback equalizer (DFE). The decision feedback equalizer operates upon separate in phase (I) and quadrature phase (Q) signals using a filter for the I signal and a filter for the Q signal without using filters for cross components of the I and Q signals. As such, the DFE contains approximately half of the multipliers and adders that are used in prior art DFEs.
The equalization filters that are used in either the DFE or FFE can be either T-spaced or 2T-spaced filters, where T is the duration of one symbol. All of the filters do not need to be the same type. For example, T-spaced filters may be used in the FFE and 2T-spaced filters may be used in the DFE or vice versa. A 2T-spaced filter accomplishes the appropriate filtering with half of the multipliers than are used in a T-spaced filter. For substantial flexibility, the filters can be structured to selectively operate in either a T-spaced or 2T-spaced mode. As such, a single filter can selectively operate as a real T-spaced filter, a complex 2T-spaced filter or a complex T-spaced filter that is half the length of the real T-spaced filter.
To establish the tap weights that achieve equalization filtering, the invention uses a blind equalization algorithm such that equalization can be achieved without using a training sequence. The blind equalization algorithm performs the well-known Sato""s algorithm to adapt the coefficients (tap weights) of the filters in the FFE and DFE to appropriately filter the received signal.
The inventive passband equalizer using T-spaced filters, 2T-spaced filters or a combination of both filter types, finds use in a digital television signal receiver such as a high definition television (HDTV) set. The inventive passband equalizer substantially reduces the number of multiplication and addition functions that need to be performed by the equalizer. As such, the complexity and therefore the cost of the receiver is substantially reduced as compared to a digital television receiver that uses a conventional baseband equalizer. Furthermore, the passband equalizer of the present invention uses blind equalization and does not rely on a training sequence to establish the tap weights of the filters.